1. Field of the Invention
The present invention relates to a communication device used in a wireless communication system, and more particularly, to a communication device handling uplink power control in a wireless communication system.
2. Description of the Prior Art
A long-term evolution (LTE) system supporting the 3rd Generation Partnership Project (3GPP) Rel-8 standard and/or the 3GPP Rel-9 standard are developed by the 3GPP as a successor of a universal mobile telecommunications system (UMTS), for further enhancing performance of the UMTS to satisfy increasing needs of users. The LTE system includes a new radio interface and a new radio network architecture that provides a high data rate, low latency, packet optimization, and improved system capacity and coverage. In the LTE system, a radio access network known as an evolved universal terrestrial radio access network (E-UTRAN) includes multiple evolved Node-Bs (eNBs) for communicating with multiple user equipments (UEs), and for communicating with a core network including a mobility management entity (MME), a serving gateway, etc., for Non-Access Stratum (NAS) control.
A LTE-advanced (LTE-A) system, as its name implies, is an evolution of the LTE system. The LTE-A system targets faster switching between power states, improves performance at the coverage edge of an eNB, and includes advanced techniques, such as carrier aggregation (CA), coordinated multipoint (CoMP) transmission/reception, uplink multiple-input multiple-output (UL-MIMO), etc. For a UE and an eNB to communicate with each other in the LTE-A system, the UE and the eNB must support standards developed for the LTE-A system, such as the 3GPP Rel-10 standard or later versions.
The UE may receive/transmit packets (e.g., transport blocks (TBs)) from/to two eNBs, when the UE is configured with a dual connectivity. Throughput of the UE is improved when the dual connectivity is operated. However, power levels for transmitting the packets to the eNBs may be scheduled (i.e., determined) by the corresponding eNBs independently. The eNB may not know the power level scheduled by the other eNB, and may schedule a power level for the UE such that the sum of the power levels is greater than the power level limit (i.e., maximum transmission power level) of the UE. The UE may not be able to transmit the packets according to the power levels, and the packets may be transmitted incompletely. Thus, UL power control becomes a difficult problem to be solved for the dual connectivity due to independent operations of the eNBs.
Furthermore, the UE may transmit the packets asynchronously to the eNBs. That is, the subframes for the eNBs may not align, and boundaries of the subframes for the eNB may locate within the subframes for the other eNB. The misalignment of the subframes may cause design of the UL power control even difficult, because the UL power control is usually performed based on a subframe basis. Thus, how to perform the UL power control when the dual connectivity is operated asynchronously is an important problem to be solved.